A processing method and a machine tool are known from U.S. Pat. No. 5,435,216 A, in which a workpiece is processed on a turret punch press at various sites in succession. For that purpose, a first site to be processed on the workpiece is arranged beneath a punch by a movement unit with a positioning movement. Before the positioning movement of the workpiece is completed, the punch is already moved towards the workpiece with an outward-stroke movement by means of a hydraulic stroke drive.
The time at which the outward-stroke movement of the punch is initiated before completion of the positioning movement of the workpiece is selected in such a manner that the punch reaches a position close to the workpiece on completion of the positioning movement. In that position, the underside of the punch is arranged at the level of the upper side of the workpiece. Starting from that position close to the workpiece, the outward-stroke movement is continued with a working stroke in which, with the punch and a die arranged on the opposite side of the workpiece cooperating with each other, a piece is punched from the workpiece.
Subsequently, the punch is moved away from the workpiece with a return-stroke movement. In a first section of the return-stroke movement, the punch is moved back to the position close to the workpiece (working return-stroke). In the working stroke and in the working return-stroke of the punch, the workpiece is not moved by the movement device. As soon as the working return-stroke of the punch has ended, another positioning movement of the workpiece follows in order for the next processing site to be arranged beneath the punch. During the positioning movement of the workpiece, the return-stroke movement is continued and the punch is moved away from the workpiece to a reversing position remote from the workpiece. Before completion of the positioning movement, an outward-stroke movement of the punch then begins again.
In practice, the initiation of the outward-stroke movement of the punch before completion of the workpiece positioning movement is also referred to as “stroke pre-triggering.”
The time of stroke pre-triggering is determined by a control unit of the known punch press as the time at which the boundary of a target region defined around the site to be processed on the workpiece arrives beneath the punch. The size of the target region has to be ascertained in advance and stored in the control unit.